This invention relates to the preparation of both positive and negative electrodes for use in high-temperature secondary electrochemical cells and batteries that can be employed as power sources for electric automobiles and for the storage of electric energy generated during intervals of off-peak power consumption. A substantial amount of work has been done in the development of such electrochemical cells and their electrodes. The cells showing the most promise employ alkali metal and alkali metal alloys as anodes, molten salt electrolytes containing the alkali metal ions and cathode materials of Groups VIA and VIIA of the Periodic Chart, the chalcogens and halogens. Sulfur, transition metal sulfides and other metal sulfides are often selected as the positive electrode reactant, while lithium, sodium and lithium-aluminum alloys are examples of reactants for the negative electrode.
Examples of such secondary cells and their various components are disclosed in U.S. Pat. Nos. 3,827,910 to Cairns et al., entitled "Homogeneous Cathode Mixtures for Secondary Electrochemical Power-Producing Cells," Aug. 6, 1974; 3,833,421 to Rubischko et al., entitled "Secondary Electrochemical Cells with a Chalcogen Cathode," Sept. 3, 1974; and 3,716,409 to Cairns et al., entitled "Cathodes for Secondary Electrochemical Power-Producing Cells," Feb. 13, 1973. Various other secondary electrochemical cells are described in U.S. Pat. Nos. 3,907,589, Sept. 23, 1975 by Gay et al. entitled "Cathodes for a Secondary Electrochemical Cell"; and 3,887,396, June 3, 1975 by Walsh et al. entitled "Modular Electrochemical Cell." Each of these patents is assigned to the assignee of the present application.
A large number of factors and variables affect the performance of high-temperature secondary cells of this type. Some very important considerations have been the reactant compositions and the structure of the electrodes. A number of fairly successful electrode compositions and structures are described in the above-cited patents and patent applications. Molten reactant materials such as sulfur in the positive electrode and lithium in the negative electrode have been sorbed or loaded along with electrolyte into electrically conductive porous substrate materials for use as the respective electrodes. Cells prepared of electrodes of this type have provided high specific energy (watt-hr/gm) and high specific power (watt/gm) but have had the disadvantage of limited life due to loss of the reactant materials from the electrodes and corrosion of the cell structure materials.
Other electrochemical cells have included electrodes having reactant materials in the form of solid particles blended with particles of electrically conductive material and electrolyte. Such type compositions have often been used in the positive electrodes with materials such as particulate transition metal sulfides, e.g. FeS.sub.2, FeS and CuS.sub.2, often blended with a particulate elecrically conductive material such as powdered carbon, carbon black or powdered iron. Compositions of these types are contained within high-temperature fabrics such as boron nitride with structural integrity and current collection imparted by embedded mesh and/or outer baskets of electrically conductive material such as iron or molybdenum.
These electrodes have functioned reasonably well in a horizontal orientation but electrode materials can be expected to sag or settle to the bottom in vertical alignments. In addition, the percentage utilization of active material and capacity at high currents has been found to be severely limited when electrodes in excess of 1/2 cm thickness are employed. Negative electrodes of such as electrochemically prepared Li-Al alloy also suffer from these limitations in capacity when increased thicknesses are provided in order to increase the loading of electrode reactant. Additional problems have arisen from the swelling and distortion of electrodes, partially as a result of inhomogeneous loading of the electrode volume with active material. This has been one of the continuing difficulties with positive electrodes employing such as FeS.sub.2 or FeS as the reactant material with embedded mesh current collectors. Also severe swelling occurs during electrochemical formation of Li-A1 electrodes from molten lithium and compressed aluminum plaques.